The present invention relates to a high-frequency heating device (microwave oven) for cooking of foodstuffs which are placed in a heating chamber, the heating and cooking operations being conducted by an electric heater and/or high-frequency generator.
Generally, a high-frequency heating device, e.g., a microwave oven, has an outer case and an inner case disposed within the outer case, which cases are substantially box-like in shape; the inner case defining a heating chamber. On the front face of the outer case, a door is provided for access to the heating chamber. An operating section including a display panel and various operating buttons is mounted on a panel to the right side of the door. Further, upper and lower electric heaters are disposed within the heating chamber. Each heater has a heat generating portion located in the heating chamber and a pair of connecting end portions attached to a rear plate of the inner case. Each connecting end portion protrudes to the rear of the inner case through an attachment hole provided in the rear plate. Outside of the right side plate of the inner case, a heat shielding plate is positioned at a predetermined distance from the right side plate. A receiving chamber is defined between the heat shielding plate and a right side wall of the outer case. The heat shielding plate prevents the heat existing at the side of the heating chamber from being radiated directly into the receiving chamber.
Within the receiving chamber, high voltage electric components for the high frequency generator are disposed. These components include a high voltage transformer, capacitor, rectifier, etc., as well as a magnetron (high frequency oscillator) for radiating high frequency energy into the heating chamber. Within the receiving chamber, there are also disposed a fan for cooling the high voltage electric components; and various controlling electric parts, such as a relay, printed circuit board, for controlling the operation of the high voltage electric components, fan, heater, etc.
Known microwave ovens with the general construction discussed above have the following operational drawbacks.
The temperature of the heat generating portion is higher than that of the connecting end portions. For this reason, in cooking, the left and right side plates; which, among the side plates of the inner case, are most closely adjacent to the heater, are likely to be heated to the highest degree. In addition, the heat radiated from the heating chamber is most intense in the vicinity of the right and left side plates. Thus, the gap defined between the right side plate and the shielding plate is made considerably large so as to enhance the effect of shielding the receiving chamber from the heat being radiated from the right side plate. In this case, however, the microwave oven as a whole becomes relatively bulky. One proposal to deal with this problem involves inserting whereby a heat insulating member is inserted between the right side plate and the heat shielding plate, to thereby increase the heat insulating effect, or utilizing high heat resistance electrical components within the receiving chamber. In either proposal, however, the cost is prohibitive.
For the purpose of avoiding the antenna effect of the lower heater when the magnetron operates, a microwave energy absorber such as, e.g., ferrite, should be provided around the connecting end portion rearwardly protruded from the rear plate of the inner case. To accommodate this energy absorber, a relatively large space for receiving the connecting end portions must be provided at the rear side of the inner case. This results in a larger oven requiring more installation space that is desirable.
Further, since a number of electrical components are received within the receiving chamber, the wiring operation between the high voltage electrical components and the controlling electrical components is difficult to perform when the device is assembled, and the maintenance and inspection thereof also become troublesome.
Since the high voltage electrical components, such as the high voltage transformer, which undergo the application of a voltage as high as approximately 2,000 V and generate heat which is relatively large in magnitude; and the controlling electrical components, which only undergo the application of a voltage as low as approximately 100 V and generate heat which is comparatively small in magnitude, are disposed within the same receiving chamber, it is likely that heat generation, leakage of the magnetic field, etc., by the high voltage transformer, magnetron, etc., respectively, will influence the controlling electric components. For this reason, it is possible that the controlling electrical components will perform erroneous operations and, at the same time, will cause a decrease in the durability of the microwave oven.